Previous methods for transmitting multiplexed analog signals from remote locations involved using an analog to-digital converter at each such remote location, and transmitting data in digital form. A digital to analog converter is needed at the data gathering control, or receiving location, for the transmitted signals. In addition, each such remote location had to have means of being addressed, and of sending -to the data gathering control- the address of its location. The accuracy of the data for such prior art systems is based on the accuracy and resolution of the analog-to-digital converter, noise, the integrity of the transmitting medium, and the accuracy and resolution of the digital-to analog converter at the data gathering control. The speed of gathering data for systems based on digital transmission is dependent on the length of the code used for the address/response (together with any bits or codes used for checking) and the sampling rate.
There are many methods for multiplexing data. These usually involve the use of modems distributed along a loop. These modems have unique addresses and are polled either sequentially or randomly. Connecting circuits sequentially is not new. For example, in telephone systems, the path from one loop to another is sequentially made via selectors. The following prior art apparatus and method, for sequentially obtaining digital information from sensors connected to remote sequencer units, is described for purposes of comparison with the present invention, which is described later.
Reference is now made to prior art FIG. 1, which shows a prior art arrangement of sequencer units 11-1, 11-2, . . . , 11-N, . . . , 11-Z. The reference characters utilized herein for sequencer units 11 and switches 15 are not intended to indicate any particular number of sequencer units or switches being used in a particular system. Each sequencer unit 11 includes a sensor 13 and a series switch 15. The dashed line in each sequencer unit 11 of FIG. 1 indicates that the series switch 15 associated with a particular sequencer unit 11 is operated by that sequencer unit to make connection to the next downstream sequencer unit. The sequencer units 11 are distributed about a volume of space, and the sensors 13 thereof monitor a certain quality of that volume, such as whether that quality exceeds a preset threshold. For example, such sequencer units 11 can be utilized to monitor whether there is a fire within the interior of a building. Sequencer units 11-1 through 11-Z are distributed along a loop of two or more wires or electrical lines 17 and 19. One end of lines 17, 19 is connected to control 21 monitoring sequencer units 11. The other end of lines 17 and 19 is connected to end of line device 23. End of line device 23 is connected across lines 17 and 19 to provide a termination, permitting continuity of current flow along the lines.
Initially, only sequencer unit 11 1 closest to control 21 is electrically connected to the control, because series switches 15 are then open. The other downstream sequencer units 11 2, etc. have one or both of lines 17 and 19 electrically open between them by action of switches 15. Periodically, control 21 will interrogate or poll each sequencer unit 11 to determine its status, confirm proper operation, learn whether a monitored condition has exceeded a preset threshold, or for any other appropriate reason. Such interrogation can be referred to as polling or as a polling sequence. One example of signals generated by the system of FIG. 1 during such a polling sequence is illustrated in FIG. 2. To initialize each polling sequence for sequencer units 11, a relatively high voltage (for example, 24 volts) is transmitted by control 21 for a relatively long duration (for example, 1 second) on the loop formed by lines 17 and 19 and end of line device 23. This transmission is identified in prior art FIG. 2 as initializing pulse IP. This sustained high voltage causes all the sequencer units 11 to interconnect by causing their associated switches 15 to close. This is a ripple effect for as the first sequencer unit 1-1 closes its switch 15-1, that sequencer unit impresses the high voltage of pulse IP onto the second sequencer unit 11-2 and causes it to close its associated switch 15-2, and so on. After sufficient time (for example, the one second duration described above) has elapsed to allow for this sequence to have propagated to the last sequencer unit 11-Z, so that its switch 15-Z has closed, this applied voltage is then reduced to a lower value also known as a working voltage (for example, 18 volts). This reduction in voltage causes all of the series switches 15 to open, so that only the first sequencer unit 11-1 is now connected to control 21, and the remaining sequencer units are reset and so disconnected from the loop. Control 21 then sends out a high voltage poll advance pulse or polling pulse Pl consisting of a high voltage for a short period, such as 24 volts for two milliseconds. Removal of polling pulse P1 causes sequencer unit 11-1 to transmit a response pulse R1.
In FIG. 2, the initializing pulse is identified as IP, the successive polling pulses for each sequencer unit 11-1, . . . , 11-Z are respectively identified as P1, . . . , PZ, and the response pulses for each such sequencer unit are respectively identified as R1, . . . , RZ. Each polling pulse P is identical, and can for example have an amplitude of 24 volts and a duration of 2 milliseconds. However, the response pulses R vary in width depending on the condition of the corresponding sequencer unit 11. The first polling pulse P1 causes the first sequencer unit 11-1 to be initialized, that is to have its switch 15-1 closed. At the end of the first polling pulse P1, control 21 waits for the sequencer unit 11-1 polled to transmit a return pulse R1. The response pulses R vary in width or duration depending on the condition of the corresponding sequencer unit. The duration of this return pulse R1 is interpreted as a normal, below normal or above normal condition at the control 21. As shown in FIG. 2, a duration of T1 indicates a normal condition, a shorter duration of T2 indicates a below normal condition, and a longer duration of T3 indicates an above normal condition. For example, a two millisecond return pulse could be interpreted as a "trouble" condition or a defective sequencer unit 11, and a five millisecond pulse could be interpreted as indicating normal condition for that unit. Also, a ten millisecond pulse could be indicative of external trouble being sensed by the sensor 13 of that sequencer unit 11 such that control 21 should generate an alarm in response thereto. Control 21 includes a counter (not shown) for keeping track of the pulse number for the polling pulse P sent, so that any abnormality is associated with the particular sequencer unit 11 being polled by that pulse. The address or number of each sequencer unit 11 is associated with its position in the loop. Thus, if the first polling pulse P1 is sent out, and the response from the first sequencer unit 11-1 is a normal duration T1 of the response pulse R1, then control 21 tags or identifies sequencer unit 11-1 in the control's memory as being of normal condition.
At the end of the first polling pulse P1, first sequencer unit 11-1 closes its switch 15-1 to connect the first sequencer unit 11-1 to the second sequencer unit 11-2. The second sequencer unit 11 2 is thereby electrically connected to the loop, so that the working voltage, and a subsequent polling pulse P2, from control 21 would be impressed on sequencer unit 11-2. The next high voltage poll pulse P2 causes the second sequencer unit 11-2 to be initialized. When poll pulse P2 is removed, it causes sequencer unit 11 2 to transmit a response pulse R2, shown in FIG. 2 as being, for example, two milliseconds (duration T2). Because the duration of return pulse R2 is below normal, control 21 tags the second sequencer unit 11-2 as abnormal. The end of polling pulse P2 causes switch 15-2 connecting the second sequencer unit 11-2 to the third sequencer unit 11-3 (not illustrated) to be closed, so that the third sequencer unit 11-3 is now connected to the loop and has the working voltage applied to it. This sequence of generation of polling pulse, generation of response pulse and closing of a switch 15 continues until all the sequencer units 11 have been polled by control 21. At the end of a pre-programmed number of polling pulses, ordinarily determined from the number of sequencer units, the sequence is reset by control 21. Control 21 then causes the loop to be reset by again sending initializing pulse IP, which reduces the voltage to the loop to open all switches 15. After resetting, the polling sequence is started again.
Although such devices have served the purpose, they have not proven entirely satisfactory under all conditions of service for the following reasons. Sequencer units cannot be polled randomly with the system of FIGS. 1 and 2. Also, the system depends on the duration of the return pulse to determine the condition of the sequencer unit. Because of the digital nature of the return pulse, the inherent capacitance of the loop wiring 17 and 19 will modify the wave shape of the return pulse, which can cause incorrect interpretation of the response signal by control 21 unless fairly wide pulse widths are used. Utilization of fairly wide pulse widths can introduce additional delay into the system and increase the time necessary to poll all sequencer units 11 in a particular loop. If there are many such units in the loop, a low-numbered sequencer unit such as 11-1 or 11-2 can detect that the threshold being monitored has been exceeded, which could indicate a hazardous condition that would not be communicated to control 21 until all other sequencer units in the loop have been polled. Since the polling pulses and response pulses are digital, a transient can cause either a false poll or a distortion or erroneous reading of a polling pulse or a response pulse. A very limited number of pulse widths can be transmitted, even if control 21 has very high resolution. As a result, very little information can be transmitted in the response pulses from the sequencer units 11. To obtain more information, a very wide range of return pulse widths would have to be accommodated by control 21, resulting in a very slow system with substantial delays between most pulses.
Furthermore, the sequential nature of the loop of FIG. 1 makes it complicated and difficult to branch from, or to add a branch to, the main loop and still keep the sequencer units 11 in sequence at control 21. Also, random polling of the sequencer units is not possible with the system of FIG. 1. Besides, it is not possible to stop the polling at any one sequencer unit, to obtain the status or level of the signal present at the sequencer unit or to obtain more information from a particular sequencer unit. The system of FIG. 1 is digital; analog data signals cannot be transmitted via such a system.